Demyx, the Melodious Nocturne (夜想のしらべ,Yasō no Shirabe?, lit. Nocturnal Poem), is Rank IX within Organization XIII. He controls water, and uses his weapon, the Sitar, as a musical instrument to make water fight for him in battle. He commands the Dancer Nobodies.

Demyx plays a minor role in Kingdom Hearts II, where he is tasked with tracking down Sora to liberate Roxas's "true disposition". Although he dislikes fighting, he is directed to use "aggression" against Sora and his companions to bring out Roxas. Demyx is a playable character in Mission Mode in Kingdom Hearts 358/2 Days.

In his own room at an unknown time, Demyx was fixing his hair in front of the mirror, compelled by something he had done when he was human. When he was finished doing his hair, Demyx picked up the sitar and strummed it. He played and played, but he was interrupted by Xaldin, who told him to keep it down because he wanted to sleep.

Annoyed, Xaldin questioned why Demyx clung to his human memories, but received no reply. Xaldin told him that Nobodies needed to sleep because a terrible force was moving that world, and they need rest to store up power. Demyx did not reply once again as Xaldin left the room, displeased.

Demyx plays a slightly larger role in Kingdom Hearts 358/2 Days, present when Xion joined the Organization. Often teased behind his back by Axel and Roxas whilst they eat sea-salt ice cream at the Station Plaza after missions, they quote how he doesn't have talent, would be "upset" if he lost his sitar, along with many other things. Demyx is the first to mention the annihilation of Organization members that took place at Castle Oblivion.

Demyx accompanies Roxas on several missions including one to the Olympus Coliseum where they take down a Flare Note, and one to Halloween Town where they tackle a Dual Blade. Demyx enjoys picking on Xion with Xigbar, and is seen playing his sitar on one of the couches in the Grey Room before many of Roxas's missions. In the manga adaption, in one of the early chapters, he is also seen playing a prank on Axel, where he puts a farting pillow on Axel's seat. When Roxas approaches Demyx, he willingly admits that he "isn't cut out for combat" and would rather perform recon.

Demyx appears in Hollow Bastion with all of the other Organization members, but like the others (except for Xigbar), he does not contact Sora. However, you can hear Demyx's higher-pitched, nervous laugh in the background.

Demyx's role in the Organization's plans is minor. He is sent after Sora on suggestion by Xigbar to find him and "liberate his true disposition", either to keep Sora on the path Organization XIII wish him to take, or try to awaken Roxas. He also contacts Hades, but no deal was made and Demyx promptly flees, saying "Run, run away!", passing by Sora and co. and leaving them puzzled at his outburst. Later, Demyx steals the Olympus Stone, and soon encounters Sora within the Underworld once more.

Although Sora doesn't understand the meaning of Demyx's objectives, nor why Demyx calls him Roxas, Demyx is unable to fulfill them and took the next ordered step, which is to attack Sora. Demyx at first only summons "forms" of water— shaped into himself and musical notes, which then attacked Sora, Goofy, and Donald—which he controls with his sitar, similar in design to the Organization symbol. He is unsuccessful in defeating Sora with his water forms, however, and escapes instead of continuing the fight. Before he takes his leave though, he begs "Roxas" to come back to the Organization, leaving Sora even more confused.

"Silence, traitor."

—Demyx when he drops his emotional ploy.

Demyx confronts Sora later in Hollow Bastion during the Battle of the 1000 Heartless, again greeting Sora in a rather friendly manner. However, Sora and his companions' response is hostile, even claiming that Demyx is unable to fight. Demyx still maintains his personality despite the insult, with only a "You shouldn't judge anyone by appearance" in a slightly deeper tone. But when Sora and co. claim that the Nobodies do not have hearts, Demyx's normal carefree air vanishes, and he silences them with a much darker tone and attacks the "traitor" again with all his strength. In the end, however, Demyx is defeated again and fatally wounded. After lamenting his loss, he fades into watery darkness, the first Organization XIII member to fall in Kingdom Hearts II and sixth member overall.

Demyx's death in the manga is depicted differently; instead of occurring in Hollow Bastion, it is set in The World That Never Was. Demyx was sent by Saix to eliminate Sora. Demyx was reluctant, albeit not because of his usual laziness, but because he believed that killing Sora also meant killing his former comrade Roxas.
When Demyx, Sora, Donald and Goofy finally meet, Sora refers to Organization XIII as "miserable members of a deplorable organization", which upsets and angers Demyx. After Donald remarks that Nobodies have no hearts, a battle unfolds, with Demyx losing his temper and blasting the three with water.

Although Demyx seems to have the upper hand, he is eventually crushed by his own spiked water, after it is frozen by Donald's Blizzard magic. Before diminishing into darkness, Demyx, laying in dismay and helplessness, tearfully laments that he does not want to fade away. Demyx's death unsettles Sora, leading him to question whether Nobodies really do lack hearts.

Demyx wears a black cloak similar in style to Xigbar's, with somewhat closer-fitting sleeves and pointed shoulder pads, though Demyx's shoulder pads are more noticeable than Xigbar's. His black gloves and boots are mostly similar to that worn by other members. He has cyan eyes and dirty-blond hair, the latter of which is quite distinctive due to its shape. Demyx's hair has an odd mullet-like style, and he has several individual bangs that fall over his face. He is rather youthful in appearance when compared to many of the other members.
Demyx is also somewhat similar to Zell Dincht from Final Fantasy VIII.

Demyx, unlike the other Organization members, has a very casual, cowardly, and lazy personality—though this may simply be an act since Nobodies are normally free of all emotion and thus cannot feel fear (though it is known that higher nobodies start to form hearts of their own, given enough time). He runs from many encounters, including ones with Hades and Sora, and in Kingdom Hearts 358/2 Days, he refuses to go searching for a giant Heartless alone. Along with Xigbar, Axel, and Larxene, Demyx is portrayed as one of the more comical members of the Organization, although unlike the other three, who earned their places in that list as a result of their sarcasm and wisecracks, Demyx holds this role because of his laziness.

Despite his laid-back personality and his tendency to sit around, Demyx seems to be on good terms with both Axel and Xigbar, calling Xigbar, "Xiggy" and is often shown talking to him in the Grey Area, and though Axel and Roxas talk about him behind his back, Axel does defend him after Roxas comments that Demyx doesn't do anything, Axel stating that Demyx specializes in recon.

Demyx seems to dislike working as much as he dislikes fighting; he even goes so far as to manipulate Roxas, dumping his missions on the younger Nobody and going off to have fun. He complains about how he "isn't cut out for combat" and also considers sweat "gross". Often, he wishes that Xemnas would not make him fight and says that he'd prefer to stay at The Castle That Never Was or at least be given a recon mission.

He also seems to dislike Larxene intensely, calling her a witch, though the feeling is apparently mutual as she has referred to him as lazy and useless. When news gets back from Castle Oblivion, he seems to be glad that she has died, showing that even he can be a bit cruel and crass without a heart. He also seems to dislike Saïx, calling him "X-face" in one of his Secret Reports, saying that Saïx constantly barks at him to get to work. However, he does show some care toward Xion, unlike many of the other members, thus demonstrating that he does have a kind side, although he thought it was "intense" when he found out that Xion was a "puppet".

Demyx is much less serious and more carefree than the other Organization members, as well as selfish, and seems to take his task of pursuing Sora lightly. He doesn't hesitate to read his orders aloud off of cue cards in front of Sora nor does he hesitate to complain about his distaste for fighting. His cries in battle after being hit are also more characteristic of "yelps" than those of the other members, and are exclamations rather than reactions. In his final battle against Sora however, he displays a side previously unseen, fighting Sora with a ferocity and wildness most unlike his 'usual' character. Yet when he is defeated, he lets out a cry of despair and disbelief, holding his head, and whimpering as he fades back into darkness.

Demyx is somewhat kinder in his manga appearances, performing actions such as giving the Olympus Stone to Sora without much of a fight, or trying to cheer up Kairi whilst she is held in a cell. He also seems to get along well with Roxas, as the two can be seen bantering if Axel and/or Xion were not at the castle.

Demyx is portrayed as being weak, cowardly, and lazy, but proves to be no pushover in the battle with Sora. Most players have trouble defeating him due to his ability to summon water clones in a time limit to defeat all of them. Demyx, being a musician, uses music to control the water around him and to summon "water forms", objects shaped out of water into musical instruments or copies of himself, which move about the battle area to hit Sora, forcing him to defeat the forms in a certain amount of time to avoid an automatic loss. He can also summon violent geysers that erupt from the ground, forming strong streams of water which damage Sora and throw him into the air. His sitar is rarely used as a weapon, but most often used to play his music instead. However, Demyx does use one move using his Sitar as a weapon, but it does little damage to Sora. Demyx does little fighting by himself, instead relying on his control of water to make it fight for him.

In Kingdom Hearts 358/2 Days, Demyx has a Limit Break called Wave Gigs. During Wave Gigs, the player must press and time the attack button in specific "beats" as Demyx jams out on his sitar, summoning up large pillars of water to rise and strike enemies. The more efficiently the button inputs are performed, the faster and more plentiful the attack will be. Enemies that are hit by the pillars take water damage. Meanwhile, Demyx's movement speed is reduced to a slow march, allowing him to slowly advance on an enemy as he attacks, but not much else. His falling is also slowed.

During Demyx's Final Limit, Demyx releases a large ball of water while jamming out on his sitar. The ball creates a watery explosion that knocks enemies back and deals water damage.

Demyx is very open about his dislike of fighting, and prefers music instead. So, he carries an enormous Sitar, named Arpeggio, a stringed Indian instrument similar to a lute or guitar, into battle. The Sitar itself is shaped like the Nobody insignia, in varying shades of blue.

With it, he can cause water to take the form of people and musical notes, making them "dance" to his music. Although not very useful in battle itself, it can cause some damage if swung at an enemy. He mostly uses its strings to control water and make it fight for him.

Airwalk produced a shoe model named Demyx (one likely focused towards women with its design), and another named Roxas.

Despite wielding water, Demyx has a weakness against Fire magic. This is because in the magic properties of Kingdom Hearts II, water is classified as a form of Blizzard magic. However, in Kingdom Hearts 358/2 Days, water and blizzard are two different types of attacks and magic.